Extremely simply text append for DISKDIR.TXT etc.

[dave-britten]

I added a really simple mechanism for appending lines of text to files on the SD card, mainly for quickly adding entries to DISKDIR.TXT and HDSKDIR.TXT directly from the console, without yanking the SD card or futzing with an XMODEM round-trip. It's like "cat >>" levels of simple; if anybody wants to supersede this with a proper vi-/WM.COM-like text editor, please do so. :)